Processes for making thin films of positive electrodes for lithium ion type electrochemical cells typically consist of mixing active electrode material which is usually in powder form with an electrically conductive material such as carbon or graphite particles and a polymer binder into an organic solvent which is then blended into an homogenized electrode slurry. The electrode slurry is thereafter coated on a current collector sheet in the form of a thin layer or film and the organic solvent is removed from the thin film by heating the thin film to evaporate the solvent. The evaporated solvent must be recovered for environmental reasons and the recovery process requires special installations to prevent the solvent in gaseous form from escaping into the environment and specialty equipments for handling and storing the used solvent which must then be disposed of. The resulting dried positive electrode thin sheet is typically porous and contain no electrolyte. The dried positive electrode thin sheet is assembled with a separator and a counterpart negative electrode and the assembly is saturated with a ionically conductive liquid electrolyte comprising a lithium salt dissolved therein to form an lithium ion electrochemical cell. The porous positive electrode is filled with the electrolyte to ensure the ionic exchange between the positive and negative electrodes.
Other coating processes for making thin films of positive electrode material for solid type lithium based electrochemical cells incorporate in the electrode mixture an electrolyte consisting of a polymer and a lithium salt. The mixture comprising an active electrode material, an electrically conductive material, a solvating polymer and a lithium salt is blended in an organic solvent into an homogenized electrode slurry. The electrode slurry is thereafter coated on a current collector sheet in the form of a thin layer or film and the organic solvent is also removed from the thin film by heating the thin film to evaporate the solvent. The positive electrode thin film thereby produced has little porosity since the electrolyte is already in the electrode and fills the space between the active electrode particles. The positive electrode thin film is then assembled with a solid ionically conductive electrolyte separator and a counterpart negative electrode to form a solid lithium based electrochemical cell.
In both cases, organic solvents are used to dilute the electrode mixture sufficiently to lower its viscosity such that the electrode mixture can be spread into thin layers. In both cases, the organic solvent must be removed prior to assembly with the electrolyte separator and the negative electrode to form an electrochemical cell. When producing in large batches or in a continuous process, the organic solvent must be recovered or treated to avoid discharge into the environment in gaseous form.
US patent application US2006/0166093 discloses a process of making positive and/or negative electrode material in which the processing solvent is water. The disclosed process utilizes a so-called water soluble synthetic rubber (SBR) mixed with a thickening agent as the electrode binder. An aqueous solution adapted for spreading is prepared which include an electrochemically active material, a water soluble synthetic rubber, a thickening agent, optionally an electronically conductive material, and water as the solvent. The aqueous solution does not contain lithium salt as the latter is known for its hygroscopic properties which would make it very difficult to dry the electrode after it has been spread into film form in order to remove the water. The film is dried for an extensive period of time (12-24 hours) to reduce the water contain to below 2000 ppm and most preferably 50 ppm. Since no lithium salt is included in the electrode mixture, the electrode film produced is porous in order to allow infiltration of a liquid or gelled electrolyte including a lithium salt to provide an ionically conductive path to the particles of electrochemically active material in the electrode. The process disclosed for making an electrode is therefore a two-step process in which the constituents of the electrode excluding the electrolyte are first mixed in water and dried and when the basic electrode is dried, the electrolyte including lithium salt is added to form a working electrode. The process disclosed is therefore specific to lithium ion type electrochemical cells and is not adapted for production of solid lithium based electrochemical cells which requires that the lithium salt be incorporated in the positive electrode prior to assembly into electrochemical cells.
Thus, there is a need for a method and a process for making electrodes for solid lithium based electrochemical cells which is cost effective and environmentally friendly.